The present invention relates to a system for reproducing signals recorded on a recording medium, and more particularly to a time base correction apparatus for correcting the jitter or time base variation caused in a reproduced signal.
One suitable system for incorporating the invention is an optical static picture recording and reproducing system employing a disk-type recording medium which is rotatable driven.
In the color video signal recording and reproducing method of this system, the color video signal can be processed without any change in the phase of a reproduced color signal even in the presence of a time base variation. One example of the method converts a 3.58 MHz color signal to a low frequency, e.g., 630 kHz signal, combines it with an FM modulated luminance signal and records the same, and another example converts a 3.58 MHz color signal to a low frequency, e.g., 1.5 MHz signal, FM modulates it along with a luminance signal and records the same.
With these color video signal recording and reproducing methods, it is well known that the reproduction of a color signal without any phase change even in the presence of time base variation can be accomplished by means of the circuitry itself and will not be described.
With the above-described color video signal recording and reproducing methods, if the reproduced video signal having time base variation is displayed on a monitor, bending or flagging of the picture is caused although no color phase irregularity occurs.
It is also known that if the frequency of the time base variation is low, there is practically no effect on the picture displayed on the monitor.
In the case of the system comprising (a) a motor for rotating a disk-type recording medium having concentric tracks (b), transducer means for recording signals on the recording medium or reproducing the signals recorded on the recording medium, (c) a control element responsive to an input control signal to reversively move the scan position of the transducer means in the track direction over the recording medium, (d) reproduced synchronizing signal detecting means for separating the synchronizing signal (e.g., the horizontal synchronizing signal of the picture) from the reproduced signal from the transducer means, (e) reference signal generating means for generating a reference signal and (f) phase comparison means for comparing in phase the signal from the reference signal generating means and the signal from the reproduced synchronizing signal detecting means and generating a control signal corresponding to a time base variation component, the conventional time base correcting method is such that if the scanning position of the transducer means is located on the recorded track, the control signal from the phase comparison means is applied to the motor and the control element so that the rotation of the motor is controlled and the control element is also controlled thereby correcting the time base variation. The relation between the control of the motor rotation and the control of the control element is such that the low frequency component of the time base variation is mainly corrected through the control of the motor rotation and the high frequency component of the time base variation is mainly controlled through the control of the control element.
With this system, each picture is recorded on one concentric track on the recording medium by modulating the intensity of a light beam generated from a light source into intensity variations in response to the video signal.
Also, sequential address signals are preliminarily recorded on the tracks and any desired one of the tracks can be sought or searched in accordance with the address signals.
One of the important functions of this system is the high speed searching of any desired one of the large number of recorded pictures.
The method of time base correction used in connection with the searching of any desired picture will now be described. When the search address corresponding to the desired picture is applied to the system, the application of the signal from the phase comparison means to the motor and the control element is stopped and the whole transducer means is moved at a high speed in the radial direction of the recording medium thereby searching the track on which the desired picture is recorded. Then, the signal from the phase comparison means is first applied to the motor controlling its rotation at a given speed and then the signal from the phase comparison means is applied to the control element thereby correcting the time base variation.
To search the adjacent track, the rotation of the motor is first controlled to a given speed and then the control element is controlled.
The reason for controlling in this way is that the recording of the system is performed intermittently and consequently there may be a case where there exists no correlation in time base variation between even the adjacent tracks.
This conventional time base correction method is disadvantageous in that a considerable time is required before controlling the rotation of the motor at a given speed due to the deteriorated response of the motor control system caused by the large inertia and thus much time is required before the time base variation is corrected and the proper video signal is generated, thereby increasing the search time.
Another disadvantage is that the motor control system is susceptible to vibrations or the like and there is the danger of variation in the motor circuit exceeding the operable range of the control element with the resulting decrease in the reliability of the time base control.